In the past, many utilities have been using high sulphur coal in their power plants for generating electricity. However, recent clean air legislation has required that sulphur dioxide emissions from power plants be reduced, and this requirement has forced many utilities to switch to natural gas or fuel oil, or to install facilities for stack gas scrubbing. Unfortunately, none of the alternatives has proven to be commercially feasible. Natural gas and fuel oil are more expensive than coal, and in many instances are difficult or impossible to obtain because of shortages. Moreover, stack gas scrubbing processes are still in the development stage.
A better approach to the solution of the problem is the use of coal gas. In the production of coal gas, the sulphur in the coal is converted to hydrogen sulphide which can be readily removed by known adsorption processes. In addition, the coal gas can be produced under pressure permitting operation economy in existing power plants and more efficient design in future power plants, as compared with the coal-burning facilities.
However, the prior art coal gas production systems require a plant with such a high capital cost that principal, interest, taxes and insurance alone mitigate any cost advantage over the use of coal in its original state. Moreover, when the additional costs of maintenance, operation and coal are added to the foregoing cost, the gas output of the prior art coal plants have a higher cost per BTU than any of the other competitive fuels. The system and apparatus of the present invention, on the other hand, has a dual advantage of low initial cost and lower maintenance. The system and apparatus of the invention is capable of producing a clean, medium BTU gas (300-500 BTU) which is much lower in cost than fuel oil and which is actually competitive with natural gas.
The apparatus of the invention includes one or more retorts, the number of retorts used being dictated by the gas requirements of any particular installation. The system permits the control of temperature and coal residence time which assists production of methane and higher hydrocarbons. Therefore, a medium BTU coal gas (300-500 BTU) can be produced in the apparatus of the invention without the prior art requirements of an air separation plant for oxygen production and without the prior art requirements for a high pressure system.
The retort used in the apparatus of the invention may be designed, as will be described, with a "wet" outer wall in which an annular chamber is provided around the reaction column. The annular chamber may contain steam tubes in which the steam introduced into the reaction column is super-heated by burning flue gas. By this construction, the walls of the reaction chamber in the retort are maintained at a higher temperature than the internal temperature of the column, so that any heat transfer through the walls is into the reaction column and not outwardly into the surrounding atmosphere. The retort itself may be modified to accommodate a wide range of different types of coal, and to produce coal gases with varying BTU values to satisfy specific requirements.
The analysis of coal gasification processes is complicated by the multiplicity of possible products and the fact that many of the chemical species do not reach their equilibrium concentration during the process. These difficulties have been overcome for practical purposes by using a simplified analysis that relies on the conservation of the mass of each element and the conservation of energy. It has been found that the assumptions necessary to analyze a coal gas process using only an equation expressing conservation of mass and one expressing conservation of energy do not greatly affect the result. The relative amounts of CO and CO.sub.2 produced do not significantly affect the lower heating value of the product gas, nor does the relative amounts of CH.sub.4 and C.sub.2 H.sub.4 and higher hydrocarbons. What is significant is the fraction of total hydrogen in the product gas that is converted to hydrocarbons, and the amount of heat added or removed from the gas during its production. The ratios of CO to CO.sub.2 and CH.sub.4 to C.sub.2 H.sub.4 are therefore fixed as representative values and the equations solved with the fraction of hydrogen converted to hydrocarbons and the heat transferred through the reactor walls as parameters. Dependent variables solved for in the analysis are the air or oxygen necessary to complete the process and the water reacted with the coal. For simplification the coal is represented by carbon, hydrogen, oxygen and ash. If the coal contains other gases such as chlorine or nitrogen, they can be considered also.
The mass conservation equations state that the weight of each element entering the process is the same as each element leaving. These are incorporated into a single equation expressing the fact that the carbon in all the carbon containing gases equals the carbon in the coal. Energy conservation is expressed by stating that the heat of formation multiplied by the mass of each gaseous compound produced must equal the heat of formation of water multiplied by the amount of water consumed plus the heat of formation of coal, less any heat added to the process through the walls. Water is introduced into the reaction column of the retort in the system of the invention as super-heated steam, so that the heat of formation of water is that for the gaseous state. An advantage of the system and apparatus of the invention is that heat is actually added to the reaction column in the retort through the walls of the retort, as compared with the prior art systems in which heat is lost through the walls of the retort.
If enough heat is added through the walls of the retort in the apparatus of the invention so as to eliminate the requirement for air, thereby producing nitrogen-free gas, the BTU value of the gas is improved 100 BTU per cubic foot, or more. Conversely, loss of heat through the walls, as is the case with the prior art reactors, costs approximately 50 BTU per cubic foot of the gas produced. The reduced heating value of the gas produced by the prior art processes as compared with the gas produced by the apparatus and system of the invention is caused by the excessive air requirements of the prior art systems. As heat is lost through the walls of the prior art retort, more air is required; and as heat is transferred into the reaction column through the walls of the retort in the apparatus of the present invention, air requirements are reduced.
Heat will be transferred inwardly from the hot walls of the retort to the reaction column by radiation as long as the walls of the retort are above the reaction temperature in the reaction column. As described above, the apparatus of the present invention uses a retort in which flue gas is used to super-heat steam. This flue gas passes along the outer surface of the walls of the retort column, so that the walls of the retort are always maintained at a higher temperature than the reaction temperature within the column.